Change to the Debian Plugins repo

Hi all,

Due to the increasing complexity of the Foreman plugins, some plugins
are no longer backwards compatible, and therefore it is no longer
sensible to have a single repo (plugins/main) for all plugins.

Effectively immediately, "main" has been renamed to "nightly" and
"1.4" and "1.3" components will appear shortly.

Please update your Apt sources appropriately.